The present invention relates to an electronic control circuit to provide efficient operation of direct current devices. More particularly, but not by way of limitation, the present invention is described herein as a control system for regulating the speed of DC motors used in electric powered vehicles.
Originally, DC motor speed control was effected utilizing resistor banks placed in series with the motor. During operation, the resistor banks are switched in and out in response to a motor speed signal. Although that method is simple in concept and easy to implement, it is extremely inefficient because a portion of the power draws from the power source is dissipated in the resistors and is wasted except at maximum motor speed. That occurs because the resistor banks may regulate the amount of power actually delivered to the motor and, therefore, motor speed, but they do nothing to alleviate the overall system load experienced by the DC battery. Thus, even though the resistor banks effectively control motor speed, they are extremely inefficient because any power not delivered to the motor is wasted as heat.
Improved systems employ silicon controlled rectifiers (SCR's) as current chopper control devices. Such systems provide a periodic on/off control that modifies the power to the drive motor and, thus, the resultant speed by variation of the "on-time" intervals. In that manner, the duty cycle (percent on-time) is adjusted. Although SCR systems are fairly effective, they are expensive, heavy, bulky, inefficient, generate excessive heat, and have poor load sharing characteristics when connected in parallel to serve large loads.
A more effective method of controlling the on/off cycle of power delivered to the motor is by using a power MOSFET (metal oxide semiconductor field effect transistor) network in place of the SCR's. A power MOSFET network is substituted for the SCR's because it takes up less space, is lighter, and is more energy efficient. One such system is U.S. Pat. No. 4,626,750 issued to Post. Post employs a power MOSFET network used as high-speed switches to deliver power to a motor using a periodic on/off cycle generated in response to an operator input. Post inputs an operator signal to a variable resistance to generate a signal representative of desired vehicle speed, and then compares that signal to a reference signal in order to determine the on/off cycle of conduction for the power MOSFET network. For example, at full speed, the power MOSFETS will conduct 100% of the time while at half speed, the power MOSFETS will conduct at approximately a 50% duty cycle. In that manner, significant improvement can be realized in battery life because with the exception of full speed, delivery of power to the motor occurs in varying durations at a fixed frequency.
Another advantage of the power MOSFET network is that it provides extremely fast switching times and allows operation at frequencies above the range of human hearing so that annoying audible tones are eliminated. However, operation at high frequencies creates voltage spikes as the power MOSFETS are switched on and off. These spikes must be suppressed to avoid component stress and degradation in overall efficiency.
A disadvantage of the Post system is the method used for voltage spike suppression. Post employs power diodes distributed through the power MOSFET network to provide for the voltage spike suppression. These diodes produce excessive heat which must be dissipated utilizing a heat sink. Unfortunately, the heat sink is also shared by the power MOSFET network and, additionally, acts as the conductor for the entire motor power control circuit. That configuration is extremely inefficient because the heat added by the diodes increases the resistance in the heat sink, thereby, inhibiting the current delivered to the motor.
A second disadvantage of the Post system is the use of two MOSFETs as switches to deliver the on/off signal to the power MOSFET network (see FIG. 5, numerals 178 and 180). During the on cycle, MOSFET 178 turns on to deliver voltage across the gates of each power MOSFET of the network, thus, turning them on. During the off portion of the cycle, MOSFET 178 turns off and MOSFET 180 turns on to ensure that the power MOSFET network is turned off. However, when MOSFET 180 is on, a path exists from +14 volts through resistors 212 and 210, diode 216, MOSFET 180, and resistor 190. Thus, a positive voltage exists across MOSFET 180 and resistor 190. That voltage also appears on the gates of the power MOSFET network. As a result, the MOSFETs are not completely turned off during the off portion of the cycle allowing a residual current flow through the MOSFETs and the motor. That current drain seriously reduces system efficiency, thereby degrading system operation and performance.
The off portion of the cycle occurs anytime the vehicle is not at full speed and includes when the vehicle is at rest without the key turned off. Only when the key is turned off will there be no off portion of the cycle. Most electric powered vehicles are simply turned on using the key and left on during the entire operating time. Thus, when a vehicle employing the Post system is operating at anything less than full speed, or more importantly, at rest, the path through MOSFET 180 will cause power to be lost.
In addition, a leakage path exists that drains battery power even with the key turned off. Referring to FIG. 1 of the Post patent, that path travels from battery 10, through resistor 104 and connection 34, across bottom electrode 26 to connection 37 and 126, and to control circuit 64. Further, referring to FIG. 5, connection 126 connects to the non-inverting input of comparator amplifier 224 through a resistor. A problem occurs because a second resistor connects the non-inverting input of comparator amplifier 224 to ground. That path bypasses key switch 100 and provides a current path to ground even when the key is off. Thus, both the constant drain along with the incomplete turn off of the power MOSFET network severely limits the battery's useful life between charges, and defeats the entire purpose of the Post system.
Accordingly, the present invention has been developed to eliminate the above problems. First, the present invention uses a separate heat sink and conductor components to provide an external and separate diode circuit which eliminates large voltage spikes without generating excessive heat. That diode circuit is composed of large capacity ultrafast switching diodes with very good load sharing characteristics necessary for voltage suppression without adding heat to the assembly. Second, the present invention has eliminated the MOSFET switches as a means to turn on and off the power MOSFET network to produce conduction and non-conduction. Finally, the leakage paths that unnecessarily drain the power source have been eliminated. The present invention, therefore, provides a design which is much simpler than conventional systems while delivering increased performance and extending battery charge life.